“Declaring” is playing the enemy’s game.
I think that's true especially for Republican candidates. The media would love nothing better than to pick apart the GOP announced field everyday, rather than talk about the mess that is the US economy.
But, when the Democrats announce early, like Obama did in EARLY 2007, it gives the media that much more time to showcase how more "appealing" the possible dem presidents are compared to the mean, incompetent Republican incumbent.
Ronald Reagan didn't declare until (I think) the 2nd week in November, even though the first primary was in Jan (actually a little earlier than what it's scheduled for next year).